Conventionally, there has been a known superconducting cable using a superconducting wire that can be in a superconducting state at cryogenic temperatures as a conductor. A superconducting cable is expected as a power cable capable of transmitting a large current with low loss, and has been developed for the practical use.
An example of a superconducting cable is illustrated in FIG. 2. A superconducting cable 10 illustrated in FIG. 2 is a single-core superconducting cable, in which a cable core 11 as the single core is housed in a thermal insulation tube 12.
The cable core 11 is composed of a former 111, superconductive conductor layers 112, an electric insulating layer 113, superconducting shield layers 114, a normal conducting shield layer 115, a protecting layer 116, and the like. The superconductive conductor layers 112 are formed by winding a plurality of superconducting wire materials spirally on the former 111. Similarly, the superconducting shield layers 114 are formed by winding a plurality of superconducting wire materials spirally on the electric insulating layer 113.
Each of superconducting wire materials for forming the layers 112 and the superconducting shield layers 114 has a laminated structure obtained by forming an intermediate layer, a superconducting layer, a protecting layer in this order on a tape-shaped metal substrate, for example. As a superconductor for forming the superconducting layer may be a RE-based superconductor (RE: rare earth element) and a bismuth-based superconductor showing superconductivity at a liquid nitrogen temperature (−196° C. in the atmospheric pressure) or more, for example. As a RE-based superconductor, an yttrium-based superconductor (Y-based superconductor, hereinafter) expressed as the chemical formula YBa2Cu3O7−δ is especially typical. In addition, a tape-shaped superconducting wire material, in which a superconductor is formed in a metal matrix, may be used. As a superconductor, a bismuth-based superconductor expressed by the chemical formula Bi2Sr2CaCu2O8+δ (Bi2212), or Bi2Sr2Ca2Cu3O10+δ (Bi2223) can be used. In the chemical formula, δ indicates a nonstoichiometric amount of oxygen.
The thermal insulation tube 12 has a double tube structure constituted of an internal tube 121 and an external tube 122. Between the internal tube 121 and the external tube 122, multilayer thermal insulator (Super Insulation) 123 is interposed and vacuumed. In addition, the outer periphery of the external tube 122 is covered by a corrosion-resistant layer 124 of polyvinyl chloride (PVC), polyethylene, or the like.
During steady operation of the superconducting cable 10, a cooling medium such as liquid nitrogen is circulated inside the internal tube 121, and thus transmitted electric current flows in the superconducting conductor layers 112 at a very low temperature.
The superconducting cable 10 is shipped from a factory and a cable line is constructed by coupling a plurality of superconducting cables 10 using intermediate connecting parts.
In a conventional superconducting cable, a vacuum part is not sectioned and vacuum parts of the terminals, cables, and intermediate connecting parts are all connected. Alternatively, as illustrated in FIG. 14, the inside of the thermal insulation tube 12 of the superconducting cable 10 is vacuumed during the manufacturing stage, and at an end of the thermal insulation tube 12, a vacuum sectioned part 129 is formed in a certain length (about 3 to 5 m, for example) (the reference sign 129a designates a partition for sectioning). Then, there is proposed a method in which when the connecting parts are assembled on a site, only the sectioned part 129 is cutoff to break vacuum, the superconducting cable 10 is coupled to a double-structure thermal insulation container 130 as an intermediate connecting part while the vacuum state is maintained in the whole of the thermal insulation tube 12, and the superconducting cable 10 is coupled to another superconducting cable using this thermal insulation container 130, thereby performing processes for laying the superconducting cables (e.g., refer to Patent Literature 1).